1. Field of the Invention
The present invention relates to a thermal printer and thermal printing method. More particularly, the present invention relates to a thermal printer and thermal printing method in which an image can be printed with agreeable quality in a form without a margin or with a very narrow margin.
2. Description Related to the Prior Art
There is a thermal printer including a thermal head in which heating elements are arranged in an array extending in a main scan direction. Recording paper is fed in a sub scan direction perpendicular to the main scan direction, so as to drive the heating elements for thermal recording. There are plural types of the thermal printer, including a thermal transfer recording and direct thermal recording. In the thermal transfer recording, an ink sheet is squeezed between the thermal head and the recording paper and heated to transfer ink from the ink sheet to the recording paper. In the direct thermal recording, a thermosensitive type of the recording paper is used, and has thermosensitive coloring layers for developing colors in response to application of heat.
In any type of the thermal printer, it is preferable to use the full width of the recording paper for printing an image without forming margins. JP-A 9-272217 discloses the thermal printer in which the thermal head including the heating elements has a greater length than the width of the recording paper. The thermal printer is provided with an edge sensor for detecting a lateral edge of the recording paper for preventing the heating elements from overheating specifically in positions outside the lateral edge of the recording paper. Thus, heating elements included in the heating elements and disposed outside the lateral edge is forcibly kept from being driven.
However, there arises a problem in the thermal printer in that coloring density of a printed image decreases toward the lateral edge of the recording paper between portions of the recording paper. The quality of printing an image will be lower because of low fidelity in reproducing the image.
Also, the recording paper may be so disposed or so shaped that one of its lateral edge comes to intersect one of the heating elements without lying on a borderline between two heating elements. The one particular heating element causes an unwanted black streak along the lateral edge, because the coloring density rises extremely in a local manner instead of the gradual decrease in the density.
In view of the foregoing problems, an object of the present invention is to provide a thermal printer and thermal printing method in which the coloring density of a printed image is prevented from having unevenness near a lateral edge of the recording material between portions of the recording material, to raise the quality of printing an image.
In order to achieve the above and other objects and advantages of this invention, a thermal printer has a thermal head including plural heating elements, wherein the heating elements are arranged in an array in a main scan direction, are driven according to image data, for recording an image to recording material moved in a sub scan direction, the recording material having a size smaller in the main scan direction than the thermal head. The thermal printer includes a position determining unit for detecting at least one lateral edge of the recording material, to determine effective heating elements in contact with the recording material among the heating elements. A data correcting unit effects a correcting process to part of the image data for heating elements of a first group included in the effective heating elements, wherein the first group contacts the at least one lateral edge and a near portion near to the at least one lateral edge with reference to the main scan direction in the recording material.
Remaining heating elements included in the heating elements but different from the effective heating elements are supplied with non-printing data by way of the image data, and are kept from being driven.
In a preferred embodiment, the data correcting unit corrects the image data by adding predetermined additional data thereto, the additional data increasing in a heat energy level toward the lateral edge between the heating elements in the first group.
In another preferred embodiment, the first group includes P heating elements, including a first heating element in a middle of which the at least one lateral edge is positioned. Q heating elements are disposed adjacent to a train of the P heating elements. The data correcting unit substitutes non-printing data for part of the image data for the P heating elements, to keep the P heating elements from being driven. The data correcting unit further corrects part of the image data for the Q heating elements by adding predetermined additional data thereto, the additional data increasing in a heat energy level toward the lateral edge between the Q heating elements.
In a preferred embodiment, P=1.
The position determining unit has plural photo receptor elements arranged in an array in the main scan direction and at a pitch equal to or less than a pitch of the plural heating elements, for detecting at least one of first and second lateral edges of the recording material.
Furthermore, a guide member guides the first lateral edge of the recording material in the sub scan direction. A pad mechanism pushes the second lateral edge of the recording material in the main scan direction, to press the first lateral edge against the guide member. The position determining unit detects the second lateral edge.
In a further preferred embodiment, the position determining unit includes first and second arrays of photo receptor elements, disposed to extend in the main scan direction, for detecting the first and second lateral edges of the recording material.
In another preferred embodiment, the plural heating elements characteristically have small resistance according to highness of temperature, and heat is caused to dissipate from the effective heating elements more quickly than from the remaining heating elements by contact with the recording material. Furthermore, a head driver applies a predetermined voltage to the heating elements. The position determining unit further comprises a temperature measurer measures temperature of respectively the heating elements. A heating time measurer measures heating time elapsed while the temperature rises to a predetermined temperature. A determiner compares the heating time between the heating elements, and determines that heating elements of which the heating time is longer are the effective heating elements.
The temperature measurer includes a measurement resistor connected with the heating elements. A voltage detector detects a connection voltage of a connection point between the heating elements and the measurement resistor. The heating time measurer includes a comparator for comparing the connection voltage with a reference voltage. A counter is started counting in response to driving of the heating elements, and is stopped from counting in response to an output of the comparator upon coming of the connection voltage down to the reference voltage.